Alternating motors

Status
Not open for further replies.
J

jksmith82

Senior Member
Location
PA
Occupation
Electrical Contractor
Good Morning Everyone,
I have (2)3 phase 5 hp motors. Presently they are on a switch that can manually turn 1 on at a time. The customer has decided they want a timer to turn on 1 pump on alternating days. Each motor is on a contractor controlled by a manual selector switch. I would like different opinions on how to achieve this. I do appreciate the input. A picture is attached.

Jim
 

Attachments

  • DF9F7642-76F6-493C-B4F8-50C0523F7218.jpg
    DF9F7642-76F6-493C-B4F8-50C0523F7218.jpg
    23.6 KB · Views: 3
You need a timer to trigger the on/off function, and a second timer to control a SPDT switch that can select between contactors in an either/or fashion
 
I believe one timer with 24 hour capability would do it. NC contacts for day one, NO contacts for day two. Repeat cycle.

Or a small PLC with an HMI, programmed for lead lag operation in case one pump trips on overload or the contactor coil fails. All sorts of good stuff and what if operation. Fun.
 
LarryFine said:
You need a timer to trigger the on/off function, and a second timer to control a SPDT switch that can select between contactors in an either/or fashion
Click to expand...
On/off function is likely already there or it runs 24/7 when "on", therefore only a timer to select which one runs today is all that is needed.

If it is a "pump down or pump up" type of station, maybe an alternating control is a consideration so each time run is called for the pumps alternate.

As mentioned best suggestions for OP involve knowing more about the application.
 
I think for this application a "time switch" would probably be better than a generic time delay relay. A time switch is designed to be accurate for at least months at a time. They typically use a quartz crystal based real time clock, or at least base it on the 60 Hz feed. Unless they specify an appropriate accuracy, a time delay relay set at 24 hours might accumulate excessive errors over a long period.

Some examples of time switches are here:

https://www.google.com/url?sa=t&sou...FjAMegQIARAB&usg=AOvVaw3saY1sxqlSAHbOZ5gn1e1g
 
I did leave out a few details I apologize for that. The pumps are going to be on for eight hours a day and alternate days. Thanks.

Jim
 
hat's what I meant by time clock. He'd need one that can be set to turn on for 24 hrs then off for 24 hrs. Either that or find a relay that changes state with each energization but ignore each de-energization, or vice versa.​
 
Jim,
With a lack of further details, we can only make suggestions based on assumptions:
  1. ASSuming that the motors run 24/7, then you will need a 48hr timer that has a "Form C" contact (Common, NO, NC) so that once every 24hrs, the timer changes state and then 24hrs later, changes BACK.
  3. If there is some kind of control device that is calling for a motor to run, i.e. a level switch, pressure switch, flow switch, thermostat etc., then you need to also determine the REASON for wanting to change motors.
  • If the reason is so that the motors get even wear and tear on them, then you want an "Alternating Relay" that is tied to that control device, so that every time the control device calls for a motor, it indexes the alternating relay so that the NEXT time it calls for a motor, it runs the other one. There are multiple forms of alternating relays out there, some are simpler to use than others. I personally like the simplest one, it used to be made and sold by Furnas, but Siemens sold that product line off to Hubbell Industrial Controls. But any alternator will work. From the looks of your photo it will not fit in that box though.
  • But an alternator will not keep track of actual run time of the motors, it just alternates their use. Generally it eventually makes the run time relatively even, but there is no actual mechanism for that because it is determined by the control device, not the relay. If you need close precision to maintaining even wear and tear, you need something more sophisticated like a micro PLC that can keep track of actual run hours. To be honest though, what most people do in that situation is just put Elapsed Time Meters on the motor starters and periodically take a look at how even the run time is. If one starts to get more run time, you artificially tweak the alternator relay to make it more even.
 
The problem with 'evening out the wear' on both motors (or whatever you're wearing out) is that they'll both wear out at the same time! Almost better to let one motor wear out faster, so you only have to replace one-at-a-time!

It's like rotating the tires on your car-- it evens out the wear, so that you'll end up buying 4 tires at once!
 
LarryFine said:
You need a timer to trigger the on/off function, and a second timer to control a SPDT switch that can select between contactors in an either/or fashion
Click to expand...
One timer and one changeover relay would provide the necessary control circuitry.
 
PaulMmn said:
The problem with 'evening out the wear' on both motors (or whatever you're wearing out) is that they'll both wear out at the same time! Almost better to let one motor wear out faster, so you only have to replace one-at-a-time!

It's like rotating the tires on your car-- it evens out the wear, so that you'll end up buying 4 tires at once!
Click to expand...

It's not really the same as car tires. Yes, they wear the same, but what if your car had 4 wheels but only two were on the pavement until they wear out? The two non-used tires will dry rot over the lifespan of the two in service.

A motor that sits in-service will collect bugs, dirt, seals may rot, oils/greases will degrade and migrate downward.

If the risk of both motors going south at the same time is too high, then have a spare motor in-stock to mitigate that risk.
 
jksmith82 said:
I did leave out a few details I apologize for that. The pumps are going to be on for eight hours a day and alternate days. Thanks.

Jim
Click to expand...
typical alternating relay setup would still run the pump that didn't run the last time each time you started it, even if you skipped a day at times.
 
Thanks kwired, I was thinking about that. I wasn’t sure.

Jim


Sent from my iPhone using Tapatalk
 
What is meant by "alternating days"
Does that mean one on one off flip flopping every 24hrs?

This is easily done with two small transistors (one N and one P, gates tied together, and a pulldown resistor on the gates), and a timer that has an output like 12vdc that drives the gates. Those two transistors then control the a relay to each pump. The timer is configured to go "on" for 24hrs every other day. Using a small programmable micro controller you can program the timer and a monitoring algorithm that can catch when a motor should be running but is not and perhaps turn on a buzzer or something.

However, this is probably not a COTS item, but when no COTS exist you have to revert to making something. I guess I am lucky to be able to design circuits and program micro controllers, and then build stuff.

If the motors have a MTBF rating, then buy a replacement motor as a spare as you get close to the MTBF #. But remember, that motor today ($$) will likely be more ($$) in the future.
 
Status
Not open for further replies.
Top